1. Field of the Invention
The present invention relates to a method and apparatus for converting halftone images, specifically for converting K-value images to N-value images (where K&gt;N), and N-value images to M-value images (where N&lt;M).
2. Description of the Prior Art
Multi-valued data is increasingly used for data output in both printing and display devices. When the image information reproduced by these devices is electronically manipulated, there is a rapid increase in the data volume. Source images are digitized for transmission, storage, and processing by facsimile machines, still-image filing systems, photocopiers, and other image processing devices. When these digitized images are reproduced by the output device without any post-digitizing processing, image deterioration becomes a problem.
This image deterioration problem is commonly resolved today by transmitting, storing, or processing the digitized source signal, and converting this digital signal to a multi-valued gradation signal at the tame of reproduction by the output device.
Various methods are used to express image density in a digital signal, including dithering and error diffusion. Halftone estimating methods are used to restore these pseudo-halftone images to multi-valued signals. Halftone estimating methods are described in Japanese patent laid-open number S58-14673 relating to a gradation signal demodulation method, U.S. Pat. No. 4,758,897 relating to a Method and Apparatus for Estimating Halftone Images from Binary Images, and Japanese patent laid-open number H2-181278 relating to a method and apparatus for image processing. With these methods plural scanning apertures are used and one of the scanning apertures is selected based on the matching/mismatching of the dot pattern observed through the scanning aperture with a reference pattern used to evaluate the text/image area. Alternatively, the scanning aperture may be selected based on the spectrum distribution of the dots in the scanning aperture. The image halftones are then estimated based on the number of black pixels in the selected aperture.
With these methods, however, text and image areas in the source image are separated based on the dot pattern in the aperture, and methods which completely change the scanning aperture based on this dot pattern evaluation tend to accentuate the effect of evaluation errors in the output image, resulting in image deterioration. A further problem is that continuous adjustment of the desired image quality is not possible.